If you are trying to have the server load a very large map, try adding this to your server.cfg and restarting the server:
If you are running a "perp" server, also contact us so that we can check to make sure that your command line has the following:
If the map still does not work, you may be missing some needed resources. Please ask us to add -condebug to your server's command line, so that the game will generate a garrysmod/console.log file that you can review in order to pin down the exact error.